Antoine Chateauminois

Role of uncrosslinked chains in droplets dynamics on silicone elastomers

We report an unexpected behavior in wetting dynamics on soft silicone substrates: the dynamics of aqueous droplets deposited on vertical plates of such elastomers exhibits two successive speed regimes. This macroscopic observation is found to be closely related to microscopic phenomena occurring at the scale of the polymer network: we show that uncrosslinked chains found in most widely used commercial silicone elastomers are responsible for this surprising behavior. A direct visualization of the uncrosslinked oligomers collected by water droplets is performed, evidencing that a capillarity-induced phase separation occurs: uncrosslinked oligomers are extracted from the silicone elastomer network by the water-glycerol mixture droplet. The sharp speed change is shown to coincide with an abrupt transition in surface tension of the droplets, when a critical surface concentration in uncrosslinked oligomer chains is reached. We infer that a droplet shifts to a second regime with a faster speed when it is completely covered with a homogeneous oil film.

Fine Tuning the Adhesive Properties of a Soft Nanostructured Adhesive with Rheological Measurements

The major objective of this article is to present recent advances in the methodology to fine tune the adhesive performance of a PSA. In addition to the so-called Dahlquist criterion requiring a low modulus, we propose two additional rheological predictors of the adhesive properties. The first one is derived from the description of the detachment of a linear elastic layer from a rigid substrate. We made an approximate extension of this analysis to the viscoelastic regime and showed that the transition from interfacial cracks to cavitation and fibrillation can be quantitatively predicted from the easily measurable ratio tan(δ)/G′(ω). If a fibrillar structure is formed, the nonlinear large strain properties become important. We showed that the ability of the fibrils to be stretched before final debonding can be predicted from the analysis of simple tensile tests. The softening, which occurs at intermediate strains, and, more importantly, the hardening which occurs at large strains, can be used to predict the mode of failure and the energy of adhesion. The use of this methodology to tune the PSA structure for a specific application has been illustrated for the special case of wb-PSA made of core-shell particles, and improved adhesive properties on polyethylene surfaces have been obtained.

Local friction at a sliding interface between an elastomer and a rigid spherical probe

Abstract.This paper reports on spatially resolved measurements of the shear stress distribution at a frictional interface between a flat rubber substrate and a glass lens. Silicone rubber specimens marked close to their surface by a colored pattern have been prepared in order to measure the surface displacement field induced by the steady-state friction of the spherical probe. The deconvolution of this displacement field then provides the actual shear stress distribution at the contact interface. When a smooth glass lens is used, a nearly constant shear stress is achieved within the contact. On the other hand, a bell-shaped shear stress distribution is obtained with rough lenses. These first results suggest that simple notions of real contact area and constant interface shear stress cannot account for the observed changes in local friction when roughness is varied.

Deformation of elastic coatings in adhesive contacts with spherical probes

This study addresses the problem of adhesive contacts between layered substrates and axisymmetric probes. A semi-analytical approach to adhesive contacts has been developed as an extension of a model recently published by Perriot and Barthel (2004 J. Mat. Res. 19 600–8) for the axisymmetric elastic indentation of non-adhesive, coated substrates. In addition to the load and penetration at equilibrium, the model allowed the derivation of the shape of the free surface in the contact zone. The validity of the approach was verified from experiments using contacts between acrylate films above their glass transition temperature (Tg) and spherical glass lenses. When the adhesive contacts were quenched below Tg, stable imprints were obtained which allowed determination of the surface deformations of the films. The latter were found consistent with the hypothesis of short range surface forces which were embedded in the contact model. Deviations from theory in the form of fingering instabilities at the periphery of the contact were observed when the confinement of the film was increased. A calculation of the stresses within the adhesive contacts indicated that these instabilities are probably driven by the release of lateral constraints within the confined films.

Surface Pressure and Shear Stress Fields within a Frictional Contact on Rubber

This paper addresses the issue of the determination of the frictional stress distribution from the inversion of the measured surface displacement field for sliding interfaces between a glass lens and a rubber (poly(dimethylsiloxane)) substrate. Experimental results show that high lateral strains are achieved at the periphery of the sliding contacts. As a consequence, an accurate inversion of the displacement field requires that finite strains and non-linear response of the rubber substrate are taken into account. For that purpose, a Finite Element (FE) inversion procedure is implemented where the measured displacement field is applied as a boundary condition at the upper surface of a meshed body representing the rubber substrate. Normal pressure is also determined in the same way, if non-diverging values are assumed at the contact edge. This procedure is applied to linearly sliding contacts as well as on twisting contacts.

Solution for the elastic field in a layered medium under axisymmetric contact loading

This study addresses the problem of the calculation of the elastic stress and displacement field within isotropic layered media in frictionless contact with rigid axisymmetric indenters. For a prescribed surface stress distribution, the integral transform approach is recalled using a matrix formulation which lends itself to generalizations to multilayered systems. It leads to an analytical solution for the Hankel transform of the elastic field which can readily be numerically inverted in the real space using available discrete Hankel transform algorithms. As an example, the shear stresses induced by the sphere indentation of a coated substrate are calculated as a function of the geometrical confinement of the contact and of the compressibility of the layer. The calculation was carried out using the surface pressure distribution provided by an exact solution to the coated contact problem. In addition, the elastic fields were also determined using an elliptic approximation of the contact pressure distribution. It is shown that the interface shear stress is strongly dependent on the details of the applied pressure profile close to the edge of the contact. In confined layers close to incompressibility, the elliptic approximation is found to result in a systematic overestimate of the interface shear stresses.

Contact compliance effects in the frictional response of bioinspired fibrillar adhesives

The shear failure and friction mechanisms of bioinspired adhesives consisting of elastomer arrays of microfibres terminated by mushroom-shaped tips are investigated in contact with a rigid lens. In order to reveal the interplay between the vertical and lateral loading directions, experiments are carried out using a custom friction set-up in which normal stiffness can be made either high or low when compared with the stiffness of the contact between the fibrillar adhesive and the lens. Using in situ contact imaging, the shear failure of the adhesive is found to involve two successive mechanisms: (i) cavitation and peeling at the contact interface between the mushroom-shaped fibre tip endings and the lens; and (ii) side re-adhesion of the fibres stem to the lens. The extent of these mechanisms and their implications regarding static friction forces is found to depend on the crosstalk between the normal and lateral loading directions that can result in contact instabilities associated with fibre buckling. In addition, the effects of the viscoelastic behaviour of the polyurethane material on the rate dependence of the shear response of the adhesive are accounted for.

Slip dynamics at a patterned rubber/glass interface during stick-slip motions

AbstractWe report on an experimental study of heterogeneous slip instabilities generated during stick-slip motions at a contact interface between a smooth rubber substrate and a patterned glass lens. Using a sol-gel process, the glass lens is patterned with a lattice of parallel ridges (wavelength, 1.6 μm, amplitude 0.35 μm). Friction experiments using this patterned surface result in the systematic occurrence of stick-slip motions over three orders of magnitude in the imposed driving velocity while stable friction is achieved with a smooth surface. Using a contact imaging method, real-time displacement fields are measured at the surface of the rubber substrate. Stick-slip motions are found to involve the localized propagation of transverse interface shear cracks whose velocity is observed to be remarkably independent on the driving velocity.

Elastic contact to nearly incompressible coatings - Stiffness enhancement and elastic pile-up

We have recently proposed an efficient computation method for the frictionless linear elastic axisymmetric contact of coated bodies. Here we give a brief description of the approach. We also discuss implications of the results for the instrumented indentation data analysis of coated materials. Emphasis is laid on incompressible or nearly incompressible materials (Poisson ratio u >0.4): we show that the contact stiffness rises much more steeply with contact radius than for more compressible materials and significant elastic pile-up is evidenced. In addition, the dependence of the penetration upon contact radius increasingly deviates from the homogeneous reference case when the Poisson ratio increases. As a result, this algorithm may be helpful in instrumented indentation data analysis on soft and nearly incompressible layers.

Friction of viscoelastic elastomers with rough surfaces under torsional contact conditions.

Frictional properties of contacts between a smooth viscoelastic rubber and rigid surfaces are investigated using a torsional contact configuration where a glass lens is continuously rotated on the rubber surface. From the inversion of the displacement field measured at the surface of the rubber, spatially resolved values of the steady state frictional shear stress are determined within the nonhomogeneous pressure and velocity fields of the contact. For contacts with a smooth lens, a velocity-dependent but pressure-independent local shear stress is retrieved from the inversion. On the other hand, the local shear stress is found to depend on both velocity and applied contact pressure when a randomly rough (sand-blasted) glass lens is rubbed against the rubber surface. As a result of changes in the density of microasperity contacts, the amount of light transmitted by the transparent multicontact interface is observed to vary locally as a function of both contact pressure and sliding velocity. Under the assumption that the intensity of light transmitted by the rough interface is proportional to the proportion of area into contact, it is found that the local frictional stress can be expressed experimentally as the product of a purely velocity-dependent term, k(v), by a term representing the pressure and velocity dependence of the actual contact area, A/A(0). A comparison between k(v) and the frictional shear stress of smooth contacts suggests that nanometer scale dissipative processes occurring at the interface predominate over viscoelastic dissipation at microasperity scale.